Many industrial operations employ cast heat resistant alloy shapes welded to other cast or wrought shapes. Additionally, it is often desirable to be able to perform repair welding on heat resistant castings either before or after some period of service. It has been found by those working in the art that heat resistant castings of less than about 8% tensile elongation present substantial welding difficulties, and those of less than about 5% present extreme welding difficulties.
While the several grades of standard heat resistant alloys (H-type) of the STEEL FOUNDERS SOCIETY OF AMERICA--ALLOY CASTINGS INSTITUTE (SFSA-ACI) have been altered to improve hot strength by fairly large additions of certain elements, Heyer, et al, U.S. Pat. No. 4,077,801, appears to have been the first disclosure of improving hot strength and rupture life of such alloys by additions to the base alloys of less than one percent each of two or more elements selected from molybdenum, tungsten, columbium, zirconium, nitrogen, titanium, cesium, lanthanum and boron. Use of such small additions is sometimes referred to as microalloying. It has been reported that microalloying in accordance with the '801 patent tends to reduce room temperature elongations about 25% to 50% below those for the untreated base alloy. While alloy types HF and HP have not been found to have presented much difficulty in microalloyed production heats, alloy types HH, HK and HT have often had such low ductilities as to present severe welding problems. Alloy type HH is probably the most widely employed of the H-type alloys, while alloy type HK is most likely second in volume of use.
It is also known that there is a close correlation between hot strength and the sum of carbon plus nitrogen content for any H-type base alloy. Thus, while there have been variations, depending upon base alloy type and other factors, about a 50% increase in hot strength is regularly attained in most H-type alloys at any given carbon plus nitrogen level by microalloying with elements from the group molybdenum, tungsten, columbium, titanium and zirconium. For, example, Post, et al., U.S. Pat. No. 2,553,330, discloses improvements in the hot workability of virtually all corrosion and heat resistant alloys by small additions of rare earth elements.
Culling, U.S. Pat. No. 5,077,006, sought to overcome the poor weldability and tendency to hot tear during casting associated with the microalloying approach disclosed in the '801 patent by microalloying with the six components, molybdenum, tungsten, columbium, titanium, zirconium and rare earth elements. While there was some improvement in properties over the '801 patent, room temperature elongations of H-type base alloys still declined with microalloying according to the '006 patent at any given carbon plus nitrogen level.
The amounts of zirconium and rare earth elements in alloys produced according to the '006 patent by ordinary air melting and pouring practices have been difficult to control. Thus, rare earth and zirconium oxide discontinuities have been observed in the microstructure of low elongation production heats.
While copper has been included in the formulation of hundreds of corrosion-resistant alloys, it has generally been considered to be detrimental or at least not beneficial to hot strength and rupture life in heat-resistant alloys. Copper is frequently found in heat-resistant alloys as a tramp, residual or unintentional element in amounts of about 0.1% to 0.4%, but is customarily either ignored or specified as a maximum of 0.5% by weight. An exception to the usual position that copper is undesirable in heat resistant alloys is disclosed by Thuillier et al., U.S. Pat. No. 4,063,934, which claims a heat-resistant alloy based on nickel and chromium, and possibly on iron, offering high oxidation, carburization and/or creep resistance at very high temperatures. In the '934 patent it is said that a nickel/chromium ratio between 1.20 and 1.40 is the main factor in the striking improvement in the carburization resistance of the alloys of that invention, but that further improvements can be achieved by further additions of the following elements whose preferred ranges are:
______________________________________ Cu 0.5 to 5% C 0.4 to 0.6% Nb (Cb) 1 to 2% (W + Mo) 1 to 5% ______________________________________
The exemplary alloys for which test data on carburization resistance is provided contain about 1% niobium plus about 1.5% of either tungsten or molybdenum, 0.4% to 0.6% carbon, and optionally 1.6% or 1.7% copper. The test data indicate good carburization resistance with further improvements provided by the addition of copper. The '934 patent also states that the disclosed alloys have high creep resistance up to very high temperatures, but no test data were provided.
In addition, although austenitic high silicon iron-nickel-chromium base alloys produced by the microalloying procedures disclosed in U.S. patent application Ser. No. 911,145, filed Jul. 9, 1992, have not presented room temperature elongation problems, improvement in their hot strength and corrosion resistance properties is desirable.